1. Field of the Invention
The present invention relates to a chemically and/or catalytically active particulate and gas filtration materials, such as those used in flue gas cleaning processes.
2. Description of Related Art
Catalytic filters are employed for a variety of gas filtering applications. Typically these filters combine some form of catalytic material (e.g., TiO.sub.2, V.sub.2 O.sub.5, WO.sub.3, Al.sub.2 O.sub.3, MnO.sub.2, zeolites, and transition metals and their oxides) within some matrix. As the gas passes over or through the matrix, contaminants within the gas will react with active sites on the catalyst to convert the contaminants to a more desirable by-product. Examples of such include:
______________________________________ Contaminant Catalyst Resulting Product(s) ______________________________________ NO.sub.x, NH.sub.3 TiO.sub.2, V.sub.2 O.sub.3, WO.sub.3 N.sub.2 + H.sub.2 O CO Al.sub.2 O.sub.3, Pt CO.sub.2 Dioxin/Furan TiO.sub.2, V.sub.2 O.sub.3, WO.sub.3 CO.sub.2, HCl O.sub.3 MnO.sub.2 O.sub.2 ______________________________________
Examples of various previous attempts to produce a catalytic filter device include those set forth in U.S. Pat. Nos. 4,220,633 to Pirsh; 4,309,386 to Pirsh; JP 4-156479 to Norio Maki; EP 0,470,659 to Ekkehard, Weber; U.S. Pat. Nos. 4,053,557 to Kageyama Yoichi; 5,051,391 to Tomisawa et al.; 4,732,879 to Kalinowski et al.; DE 3,633,214 A1 to Dr. Hans Ranly.
In certain cases (e.g., U.S. Pat. No. 4,220,633 and U.S. Pat. No. 4,309,386) the filters have to collect substantial amounts of dust, such as that generated in a combustion process. After short collection times of between 1 minute and 6 hours, a layer of collected dust on the dirty side of the filter material increases the pressure drop across the filter and the filter has to be cleaned. (In many cases in situ.) During this cleaning cycle (e.g., a high energy air impulse system, a shaker system, a reverse air system, etc. ), the outer dust layer falls off and a new filtration cycle can begin. Most catalytic filter materials today of which none are commercially available constitute a mesh of a regular woven or non-woven filter material in which the catalytically active particles are inserted as a foreign body. During the cleaning cycle, in which the filter material is exposed to high energy input and flexing, these particles are believed to abrade the host fibers at the fiber interception points and degrade the life of the filter.
Furthermore, inserted catalytic particles have the disadvantage that they increase the pressure drop of the filter material. In a filter which is used for particle collection, an optimal percentage of the filter will be occupied by fibers. If less fibers are used, the filter becomes weak and particulate collection will decrease. If more fibers are used, the filter will become stronger and collect dust particles at a higher efficiency but the pressure drop across the filter will increase above tolerated levels. Since catalytic particles on the surface of fibers will not increase fiber strength but rather fiber diameter, one will have to use at least the same amount of fibers as used for the original filter to obtain sufficient strength. In this case, the pressure drop increases significantly. On the other hand, if less fibers are used to keep pressure drop consistent, the resulting filter will be weaker. In general it can be said that the more volume in a filter is occupied by noncatalytic constituents, the lower the catalytic activity per unit volume of the filler or the higher the pressure drop across the filter. In addition, the contact of the pollutant dust with the catalyst particles in the filter medium will decrease catalyst activity due to catalyst pore or active site clogging. In cases where glue is employed to help anchor the catalyst in place within the material, the glue tends to clog active sites on the catalyst and diminish its effectiveness. In cases in which catalyst particles are glued to solid surfaces, gases can only access the catalyst from the side which is directly facing to the fluid stream.
A number of other instances (e.g., Japanese Patent Application JP 4-235718 to Vilene Co., Ltd.) employ integrated catalysts and support matrix. While abrasion from loose particles can be reduced or avoided with this approach, these devices continue to have significant problems. First, many of these materials are relatively weak and tend to provide inadequate catalyst retention and/or are easily damaged during handling and use. This condition usually worsens as the quantity of catalyst is increased in the matrix.
Second, the catalytic filter must be thin and open enough to assure that gas can readily reach the active catalytic sites. Unfortunately, providing a thin and open structure decreases the strength and integrity of the filter material even further. While reinforcing materials or thicker or denser materials might be employed in the filter to increase its strength, the filter will undergo a resulting decrease in gas removal efficiency since there will be fewer fully exposed active catalytic sites for gas contact. Furthermore, denser or thicker material will cause an undesirable increase in the pressure drop through the material. These problems are particularly evident in Japanese Patent Application JP 4-235718 to Vilene, where it is taught that the catalytic material may need to have holes punched into it in order to produce adequate flow-through properties. Of course the use of through-holes is not entirely acceptable since gas flowing directly through macroscopic holes in the material will not contact any catalyst.
Third, contamination is a serious problem with virtually every previous catalytic filter device. Although by definition a catalyst is not consumed during the catalytic reaction, until the present invention catalytic filters may have limited operating lives due to particle contamination in a fluid stream (e.g., fine dust particles, metals, silica, salts, metal oxides, hydrocarbons, water, acid gases, phosphorous, alkaline metals, arsenic, alkali oxides, etc.). Over time, these aerosols tend to become embedded within the filter matrix, thus blocking the pores of the catalyst and, therefore, minimize the surface area and access to the active sites of the catalyst. Unless these particles can be shed from the filter, the filter will rapidly diminish in efficiency until it must be replaced. As has been noted, a variety of cleaning apparatus exists to remove dust from filter apparatus (e.g., shaker filter bags, back-pulse filter bags and cartridges, reverse air filter bags, etc.), but these devices are not expected to be particularly effective at removing dust from current integrated catalytic filter materials. This is due to the filter's overall weakness, preventing its rigorous handling; and the intricacies of the filter structure, making it very difficult to remove particles from the matrix once they have become embedded therein.
Accordingly, it is a primary purpose of the present invention to provide a catalytic filter material that is effective at catalytically converting contaminants in a fluid stream. Fluid streams in this invention are gas and liquid streams.
It is a further purpose of the present invention to provide a catalytic filter material that has improved strength and a more open structure over existing catalytic filter designs. Within the new filter structure, pollutant molecules can access the catalyst particles from all sides.
It is yet another purpose of the present invention to provide a catalytic filter that can be effectively cleaned, with minimum contamination of the catalytic particles, so that the filter has an extended effective operating life.
These and other purposes of the present invention will become evident from review of the following specification.